The invention refers to a method of separating solid and/or liquid particles and/or polluting gas from a gas stream by utilizing a washing liquid which is superfinely distributed in a gaseous phase and introduced in the gas stream, as well as to an apparatus for carrying out the method.
Great technical problems are experienced during separation of superfine dust at a grain size of less than 0.5 .mu.m. Dust particles of this size are created by various technical processes, less because of comminution processes but mainly because of a buildup from the gas phase via condensation, coagulation and/or desublimation.
Chemical analysis of sintered dust which remained in the exhaust gas after a conventional dust removal revealed that it can contain a very high degree of alkali salts which are formed during the sintering process from the gas phase. This high fraction of salt aerosols leads to a very great fineness of the dust, with the dust containing a high fraction of particles which have an aerodynamic diameter of less than 0.2 .mu.m.
It is known from the literature that a separation minimum exists for a grain size range of 0.1-0.3 .mu.m since the forces of inertia decrease superproportionally with decreasing particle size and the diffusion forces are not yet sufficiently effective.
Greater problems are also experienced when significant differences in concentration, quantity and physicochemical properties of the aerodispersion occur during continuous operation.
Besides conventional dry filters, dust scrubbing proved the leading method for separating fine dust as well as liquid and gaseous pollutants.
The effectiveness of the wet scrubber is based on a binding of pollutants in a liquid phase during washing of the aerodispersion. The reason for the conversion of the pollutants into the liquid phase are the forces of inertia and diffusion forces as well as potentially electrical forces.
Conventional methods such as described in DE-OS 37 14 749 and in DE-PS 27 46 975 employ for contacting dust and water mist droplets pipes which include cross-sectional constrictions, such as e.g. a Venturi segment, for improving the flow rate of the raw gas. A cleansing liquid is added into this pipe segment or directly thereafter in order to create an intimate mixture between gas and mist droplets through strong turbulences during the pressure change of the gas stream.
Drawbacks include the high pressure drop so that the energy consumption increases, and the extremely short contact period (&lt;0.4 sec) so that the fractional degree of separation is adversely affected.
U.S. Pat. No. 4,067,703 discloses a dust scrubber in which the washing zone or spraying zone essentially includes a conduit of a diameter and length which are dimensioned such that a steady turbulence as well as a relatively long retention time for the contact between the solid particles and the wash water droplets are created. By means of a nozzle, wash water is very finely distributed in the gas stream which is to be freed from dust. The dust-laden wash water is separated in a separator which is built around a centrifugal blower and includes an immersion tube for the sludge drain.
This nozzle is arranged in the gas stream, with a minimal distance being maintained between the nozzle and the blower so that the spray cone completely fills out the interior of the pipe. This US-PS refers to an example with a ratio of the distance between nozzle to blower: pipe diameter=5:1. Upon using two-component nozzles (compressed air/water), droplet sizes between 5 and 100 .mu.m are possible, with the greatest portion measuring between 10 and 30 .mu.m.
Following the spray zone is the low-pressure blower which serves not only for transporting the dust-gas mixture but expands the contact space for impacting of the dust particles beyond the area of the two-component nozzle. At the same time, this blower is supposed to promote the coagulation of the water droplets. Dust collected together with water is discharged within the blower through an immersion tube with water seal into a sludge tank.
The separation curves measured for this dust scrubber (see FIG. 6 of the US-PS) have a minimum at about 0.4 .mu.m particle size. This points to the existence of dust particles which, on the one hand, are too small for inertia effects and, on the other hand, too large for diffusion effects.
This dust separator has thus the following drawbacks:
The operation is possible only with clean water, resulting in a high waste water accumulation
the gas throughput is limited
the material strength is subjected to high stress through corrosion and erosion
the flexibility in the gas throughput and dust throughput is low
the pressure loss is high.
DE-AS 25 20 957 describes a method of purifying sintered exhaust gases, with a first partial stream of the exhaust gases which has a small content of gases or vapor-like pollutants being collected from air boxes below the first and last part of the sintering machine and being freed essentially from dust in a first electrostatic gas purification. A second partial stream with a higher pollutant load is withdrawn from intermediary located air boxes and freed from dust in a second electrostatic gas purification, thereafter freed from polluting gases or vapor in a gas scrubber and then released into the atmosphere via a chimney.
This combination has however the following drawbacks:
Tests conducted by the inventors with the known plant showed that the curve for the fractional degree of separation has a clear minimum at a particle diameter of 0.2 .mu.m.
The separated collection, drain and purification of two differently laden exhaust gas streams significantly complicates the apparatus.